For producing polycarbonates, known are a method of directly reacting an aromatic dihydroxy compound such as bisphenol A or the like with phosgene (interfacial polycondensation), and a method of transesterifying an aromatic dihydroxy compound such as bisphenol A or the like with a dicarbonate such as diphenyl carbonate or the like in a melt or solid phase (melt polymerization, solid-phase polymerization).
The interfacial polycondensation method is problematic in that it requires toxic phosgene and the chlorine-containing side products formed corrode the apparatus used. On the other hand, the melt polymerization method in which the monomers are reacted for a long period of time at high temperatures generally falling between 280.degree. C. and 310.degree. C. is also problematic in that the polycarbonates produced are inevitably colored and could not have a high molecular weight.
To solve the problems with the melt polymerization method, one proposal was made in Japanese Patent Laid-Open No. 208823/1996. The method proposed comprises polymerizing the prepolymer for the intended polycarbonate in a poor solvent gas stream in the presence of a nitrogen-containing organic basic catalyst. In that method, the polycarbonate produced could have good quality but could not have a sufficiently increased molecular weight. The method could not still solve the outstanding problem of how to produce high-molecular-weight polycarbonates.
To produce high-molecular-weight polycarbonates, another method was proposed, which comprises preparing a polycarbonate prepolymer followed by polymerizing it in a solid phase and in which the prepolymer being polymerized is in a swollen solid phase in a swelling solvent gas stream (Japanese Patent Laid-Open No. 235368/1997). In that method, however, polycarbonates having a sufficiently increased molecular weight could not still be obtained. What is more, the method is problematic in that, when the polymerization temperature is further elevated so as to increase the molecular weight of the polycarbonate being produced, then the polycarbonate produced dissolves in the swelling solvent used.
In the laid-open patent specification, also proposed was solid-phase polymerization of the prepolymer in an atmosphere of nitrogen gas or lower hydrocarbon gas. However, this is not favorable since the efficiency of trapping the phenolic side products formed during polymerization is poor and the side products are difficult to separate, remove and recover.
The object of the present invention is to solve the problems with the conventional polycarbonate production methods as above and to provide an efficient process for producing high-quality polycarbonates having a high molecular weight.